The present invention relates generally to diffraction gratings made using polymer dispersed liquid crystal (PDLC) materials, and, more particularly, to thick volume PDLC diffraction gratings with high switching contrast.
Transmission gratings made using polymer-dispersed liquid crystal, or PDLC, materials have been shown useful for the construction of low loss electrooptic switches. For example, Stone et al. in Proc. SPIE 4112, 38-47 (2000) describe optical switches based on cascades of electrically switched PDLC transmission gratings. Such gratings operate in the Bragg regime and an incoming optical beam is switched between the zero- and first-diffracted orders via an applied electric field. More specifically, a PDLC grating diffracts an incident optical beam unless a sufficiently strong electric field is applied across it, in which case the grating is rendered transparent, or clear.
The PDLC gratings described by Stone et al. can be further characterized as phase gratings, because the PDLC morphology includes switchable nematic liquid crystal content contained in isolated or interconnected cavities that are much smaller than the wavelength of the optical beam being switched. This results in gratings with microsecond switching time and low intrinsic scattering loss at wavelengths in the near infrared spectral region. The article by Stone et al., though, does not provide specific instruction on how PDLC phase gratings may be optimized for high contrast performance. As highlighted by Stone et al. in Proc. SPIE 3463, 86-97 (1998), high contrast gratings are essential for building switches with low crosstalk.
In another example, Sutherland et al. in U.S. Pat. No. 5,942,157 describe how PDLC materials may be formulated and applied to the fabrication of electrically switchable diffractive and refractive devices. In this reference, a description of the PDLC grating switching mechanism is provided; the condition for optical transparency is described as requiring the polymer index of refraction be equal to the ordinary refractive index of the nematic liquid crystal. This description is simplistic and fails to adequately describe salient features of PDLC phase grating operation; especially, the operation of PDLC transmission gratings with p-polarized optical beams. As a result, it provides no teaching on the fabrication of gratings with high contrast.
In yet another example, Bunning et al. in Polymer 36(14), 2699-2708 (1995) describe the operation of PDLC transmission gratings. As in the previous example, a simplistic description is provided to describe the optically clear state of the gratings; no instruction for the fabrication of high contrast PDLC gratings is offered.
In U.S. Pat. No. 4,688,900, Doane et al. describe PDLC light modulating materials that can be switched from substantially scattering to substantially clear states using either an applied electric field or thermally, by heating the PDLC materials. Subsequently, in U.S. Pat. No. 4,890,902, Doane et al. describe PDLC materials with selectable viewing angles. In particular, formulations are described that allow the fabrication of PDLC films that can be switched to a clear state for a selected viewing angle, or range of viewing angles. This is accomplished by selecting or adjusting the polymer constituent of the PDLC according to the index of refraction. The descriptions offered by Doane work for scattering mode PDLC devices; however, it cannot be extended to PDLC phase gratings to devise a grating with high contrast operation.
Therefore, it is an object of this invention to provide PDLC transmission gratings with high switching contrast.
It is a further object of this invention to provide high switching contrast PDLC transmission gratings by optimizing the optical properties of the PDLC constituents.
It is still a further object of this invention to provide high switching contrast PDLC transmission gratings by optimizing the selection of switching field and Bragg angle.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described herinbelow.
The present invention relates to PDLC transmissive phase gratings, also referred to as optical switches, which utilize nematic liquid crystal materials confined in a polymer matrix. Such gratings can be used as, but are not limited to, switchable beamsteering devices for free-space optical beams. In order to be useful for telecommunications and other applications, it is desirable that the PDLC gratings operate in the Bragg regime and switch between diffracting and non-diffracting states via an applied electric field. Further, it is desirable that the gratings provide high contrast and low loss.
In this invention, salient PDLC grating parameters are described in the context of providing optimized PDLC transmission gratings with high contrast switching. The interplay between the PDLC materials optical properties, the grating period, and the applied electric field strength are also elucidated in this invention, as is the impact of these three grating properties on switching contrast.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description.